Scientists at Baylor College of Medicine tested antioxidant nanoparticles created at Rice University and found they were taken up by cells in the immune system.

That could be a plus for patients with autoimmune diseases like multiple sclerosis.

Baylor's Associate Professor Dr Christine Beeton said: "Placed just under the skin, the carbon-based particles form a dark spot that fades over about one week as they are slowly released into the circulation.

"We saw it made a black mark when we injected it, and at first we thought that's going to be a real problem if we ever take it into the clinic

"But we can work around that. We can inject into an area that's hidden, or use micropattern needles and shape it.

"I can see doing this for a child who wants a tattoo and could never get her parents to go along.

"This will be a good way to convince them."

The study published Scientific Reports noted T and B lymphocyte cells and macrophages are key components of the immune system.

In many autoimmune diseases such as multiple sclerosis, T cells are the key players.

But one suspected cause is that T cells lose their ability to distinguish between invaders and healthy tissue and attack both.

In tests at Baylor, nanoparticles were internalised by T cells, which inhibited their function, but ignored by macrophages.

Lead author graduate student Redwan Huq said: "The majority of current treatments are general, broad-spectrum immunosuppressants

"They're going to affect all of these cells, but patients are exposed to side effects ranging from infections to increased chances of developing cancer.

"So we get excited when we see something new that could potentially enable selectivity."

Since the macrophages and other splenic immune cells are unaffected, most of a patient's existing immune system remains intact, he said.

In Mice experiments the soluble nanoparticles were not toxic, were slowly taken up by the T cells where they collect and inhibit the cell's function but did not remain in T cells and dispersed within days after uptake by the cells.

Prof Beetin said: "That's an issue because you want a drug that's in the system long enough to be effective, but not so long that, if you have a problem, you can't remove it."